This invention relates a liquid crystal display device, and more particularly to a technique useful for a circuit for supplying a video signal voltage to each pixel. Recently, liquid crystal display devices have been widely used in small-sized display devices, display terminals for office automation equipment and the like. Basically, a liquid crystal display device includes a liquid crystal display panel (also called a liquid crystal display element or a liquid crystal cell) composed of a pair of insulating substrates at least one of which is made of a transparent plate, a transparent plastic plate or the like, and a layer of liquid crystal composition (a liquid crystal layer) sandwiched between the insulating substrates.
The liquid crystal display devices are divided roughly into the simple-matrix type and the active matrix type. In the simple-matrix type liquid crystal display device, a picture element (hereinafter a pixel) is formed by selectively applying voltages to pixel-forming strip electrodes formed on both of the two insulating substrates of the liquid crystal display panel, and thereby changing orientation of a portion of liquid crystal molecules of the liquid crystal composition corresponding to the pixel. On the other hand, in the active-matrix type liquid crystal display device, the liquid crystal display panel is provided with signal lines, scanning lines, pixel electrodes, and active elements each associated with one of the pixel electrodes for pixel selection which are formed on one of the substrates, and a pixel is formed by selecting the active element associated with the pixel and thereby changing orientation of liquid crystal molecules present between a pixel electrode connected to the active element and the reference voltage electrode associated with the pixel electrode.
The liquid crystal display device of the active matrix type having an active element (a thin film transistor, for example) for each of pixels and switching the active elements is widely used as a display device for notebook personal computers and the like. Among the liquid crystal display devices of the active matrix type, a liquid crystal display device of the so-called driver-circuit-integrated type is known which has a pixel-electrode-driver circuit fabricated on a substrate on which the pixel electrodes are fabricated. The liquid crystal display devices are operated by AC driving which inverts the polarity of a voltage applied across the liquid crystal layer periodically. The object of the AC driving is to prevent deterioration of the liquid crystal composition caused by DC voltage application across the liquid crystal layer.
For the active matrix type liquid crystal display device which applies voltages between the pixel electrodes and the reference electrode, one of the AC driving methods is such that a fixed voltage is applied on the reference electrode and the pixel electrodes are supplied alternately with positive-polarity and negative-polarity signal voltages. However, in the above AC driving method, the driver circuit needs to be a high-voltage circuit capable of withstanding a voltage difference between the maximum positive value and the maximum negative value of the pixel electrode voltage. A control signal (a scanning signal) for on-or-off control of the thin film transistors also need to be a high voltage.
Recently, the number of steps of a gray scale displayed in the liquid crystal display devices has been increasing to 64 or 256. There is also demand for high-definition liquid crystal display devices having a larger number of pixels. When the number of steps of a gray scale to be displayed, the circuit becomes large in scale, and when the number of pixels is increased, the driving circuit for supplying signals to the respective pixels is operated at high speed, and an area which each of the pixels can occupy is reduced. On the other hand, in high-voltage circuits it is difficult to miniaturize their circuit elements, and as a result the scale of the circuits becomes larger. Especially in small-sized liquid crystal display panels, even when there is a demand for an increase in the number of pixels, it has been difficult to fabricate a structure such as a high-voltage active element within a limited area of each pixel. Further, in a liquid crystal display device of the driver-circuit-integrated type having a driver circuit incorporated into its liquid crystal display panel, a problem arises in that since the area occupied by the driver circuit increases, the liquid crystal display panel becomes large-sized. Moreover, in the high-voltage circuit, there is a problem in that, because the area of its electrodes and others are increased, the resultant increase in their capacitive components makes it difficult to operate the driver circuit at high speed, and also increases its power consumption.